Sophy Warner wanted to vote for third-party U.S. presidential candidate Jill Stein. But she worried that her ballot, cast in the swing state of Ohio, might help Republican Donald Trump capture the White House. Through the website "Trump Traders," the 20-year-old biology student at Cleveland State University got in touch with Marc Baluda, 44, a Republican corporate lawyer in California who opposes Trump's candidacy and planned to vote for Democratic nominee Hillary Clinton.

The two strange bedfellows made a deal worthy of congressional horse-trading: Warner would vote for Clinton in Ohio, where polls show a tight race, while Baluda would cast a ballot for the Green Party candidate Stein in California, where Clinton is assured of winning the state's electoral votes.

Tens of thousands of voters, the vast majority seeking to prevent a Trump presidency, have signed up on "vote-swapping" exchanges in advance of Tuesday's Election Day. There is no way to verify the ballots are cast as agreed, though some people are taking "ballot selfies" in states where such photos are legal.

The swaps take advantage of a unique feature of U.S. presidential elections. The winner is decided not by the national popular vote. Rather, the outcome depends on what are known as electoral votes, which are awarded to the victor of each state's presidential election, with rare exception.

The overall electoral vote winner becomes president, and the national contest thus often comes down to votes in a handful of states.

"Swing states" such as Ohio are hotly contested because their voters can swing either to Republicans or Democrats year after year and so play a decisive role. By contrast, pollsters view states such as California as reliably Democratic.

40,000 MATCHES

Trump Traders had matched 40,000 voters as of Monday, according to co-founder John Stubbs. Although that may be a small fraction of the electorate, a few hundred votes could make a difference in a state where the race is close.

The practice appears to be legal. In 2007, the 9th U.S. Circuit Court of Appeals in San Francisco ruled that swapping votes is a protected form of free speech, even if some disagreed with the tactic.

Vote trading first gained attention in 2000, when some voters sought to ensure Ralph Nader, the Green Party candidate, did not siphon off enough support from Democrat Al Gore to hand the election to Republican George W. Bush. The so-called "Nader Traders" failed when Bush famously won the election after capturing Florida by only 537 votes. Nader drew more than 97,000 votes there.

Stein and Libertarian nominee Gary Johnson together are drawing nearly 7 percent in opinion polls, far more than normal for those parties and enough to raise the specter of another Nader-style outcome in 2016.

The digital exchanges seek to solve a quadrennial conundrum for voters "trapped" in one of the 40 or so noncompetitive states: how can I make my vote count? For supporters of third-party candidates like Stein and Johnson who have almost no chance of capturing electoral votes, however, all that matters is their raw national totals.

That difference is what allows the type of vote trading that occurs on Trump Traders and #NeverTrump, a mobile app launched this fall by Silicon Valley entrepreneur Amit Kumar. "Living in California, our votes aren't that important in determining who wins," he said in a phone interview. Kumar said the app has been downloaded 20,000 times, with around 8,000 active users.

For Republican voters like Baluda, even saying aloud that he is supporting Clinton is difficult. But he said he had no regrets about trying to maximize the power of his vote by commoditizing it. "Votes do matter, and Floridians found that out 16 years ago," he said.